1. Field of the Invention
The present invention relates to an organic-inorganic (or organic-silicate) complex composition for coating a metal surface, and more particularly to an organic-inorganic complex composition for coating a metal surface that largely improves the surface protecting ability and the corrosion resistance of a steel sheet (or plate) treated by chrome or phosphoric acid and plated with an alloy of zinc-nickel, zinc-iron, zinc-aluminum, etc.
2. Description of the Prior Art
Research and development in improving the corrosion resistance in steel sheet by utilizing an alloy technique using metals such as zinc, nickel, chrome, manganese, aluminum, copper, cobalt, etc. have been continued. However, it is very difficult to obtain a sufficient corrosion resistance. Moreover, a stainless steel sheet having a good corrosion-resistance is very expensive, so the usage thereof is limited. In North America and Europe, since a lot of salts for defrosting and removing snow are used for preventing a frosting during winter, an anticorrosive steel sheet applied to the body of an automobile, becomes rigid. In order to solve this problem, a new method using an organic coating agent has been proposed. The conventional organic coating agent for the anticorrosive steel sheet is coated to a thickness of around 15 .mu.m, and is called a thick film-type organic coating agent. However, the problems of powdering, low continuous weldability, sacrificial ability, etc. appear during a molding process.
Recently, a thin film-type complex coating agent which gives the film thickness of 1 to 2 .mu.m, has been developed. This thin film-type complex coating agent has good corrosion-resistance, an improved continuous working characteristic, an anti-powdering characteristic, a good electrodeposition coating characteristic, etc. when compared with the above-mentioned thick film-type organic coating agent.
As an example of the thin film-type coating composition, disclosed in U.S. Pat. No. 5,213,846 (issued to Tsuneta et al.) is a corrosion-resistant coating composition. This composition comprises (i) a bisphenol type epoxy resin comprising bisphenol skeletons and epichlorohydrin skeletons and having at least two epoxy groups per molecule, and comprises (ii) silica particles. The bisphenol skeletons comprise bisphenol A skeletons and bisphenol F skeletons.
The coating method of the thin film-type complex coating agent includes the steps of coating a steel sheet plated with an alloy such as zinc-nickel, zinc-iron and zinc-aluminum, with 20 to 100 mg/m.sup.2 of a surface treating agent consisting essentially of chrome or phosphoric acid, coating the complex coating agent to the thickness of 5 .mu.m or less, and continuously coating, curing and drying at the material temperature of 100 to 250.degree. C. This method is called a PCM coating method.
In the conventional thin film-type complex coating agent, epoxy resin, acryl resin, urethane resin, modified epoxy resin, modified acryl resin, etc. are used as a resin (binder). However, these components have the defects of weak adhesiveness to the material and not particularly good curing property, corrosion-resistance, continuous weldability, anti-powdering characteristic, alkaline-resistance and electrodeposition. Especially, at a high curing temperature of about 200.degree. C. or over, an energy efficiency is not good and the properties of a steel sheet having high tension are deteriorated.
In addition, if the coating agent of the thin film-type is not sufficiently cured, the thus obtained coating layer may be exfoliated or a pre-treated chrome solution may liquate out during the removing process of an alkaline compound and a rinsing process carried out after the curing. Therefore, the corrosion-resistance of the steel sheet may be deteriorated or an important environmental problem may be induced.
Accordingly, the present inventors continue to improve the defects of the conventional thin film-type complex coating agent and develop a water-soluble thin film-type organic-inorganic complex coating composition which has a good curing ability at low temperature, alkaline-resistance, corrosion-resistance, a good adhesiveness to the material, a good electrodeposition coating characteristic, an anti-powdering characteristic, continuous weldability and a good work efficiency with respect to coating. This organic-inorganic complex coating composition uses acrylic or vinyl modified epoxy resin as a main component and is manufactured considering the environmental problem and the energy problem.